public class Main {
    public static void main(String[] args) {
        System.out.println("Hello world!");
    }
    //动态规划
    public int tribonacci1(int n) {
        if(n == 0) return 0;
        if(n == 1||n == 2) return 1;
        int[] dp= new int[n+1];
        dp[0]=0;
        dp[1]=1;
        dp[2]=1;
        for (int i = 3; i <= n; i++) {
            dp[i]=dp[i-1]+dp[i-2]+dp[i-3];
        }
        return dp[n];
    }

    //使用滚动数组优化
    public int tribonacci(int n) {
        if(n == 0) return 0;
        if(n == 1||n == 2) return 1;
        int a=0,b=1,c=1,d=0;
        for (int i = 3; i <= n; i++) {
            d=a+b+c;
            a=b;b=c;c=d;
        }
        return d;
    }
    public int waysToStep(int n) {
        if(n == 1||n == 2) return n;
        if(n == 3) return 4;
        int MOD=1000000007;
        int[] dp= new int[n+1];
        dp[1]=1;dp[2]=2;dp[3]=4;
        for(int i=4; i<=n; i++){
            dp[i]=((dp[i-1]+dp[i-2])%MOD+dp[i-3])%MOD;
        }
        return dp[n];
    }
}